This proposal, which is responsive to the NIH Program Announcement on Non-Biodefense Emerging Infectious Diseases Research Opportunities, is concerned with Lyme borreliosis. This disease is caused by B. burgdorferi (Bb) and is the most commonly reported arthropod-borne illness in the US and Europe with the number of reported infections raising each year. The spirochetes cause persistent infections and chronic disease with bouts of arthritis and carditis in untreated patients. Our long-term objective is to delineate the mechanisms with which Bb evades effective immunity during persistent infection of fully immunocompetent hosts. Previous studies have identified antibodies as a main immune mediator of disease resolution (but not clearance of the spirochete) and passive protection. While antibodies in early infection are strongly functional with respect to disease-resolution and passive protective capacity, the protective capacity wanes as disease progresses. The objective of this proposal is to assess the mechanisms underlying the regulation of the Bb- specific humoral response, to ascertain whether strengthening of humoral responses could be exploited for therapeutic or prophylactic use. Based upon preliminary findings, we aim to test the hypothesis that Bb- specific B cells provide antibody responses by short-lived plasma cells at the expense of higher affinity B cells and long-lived plasma cells due to a lack of functional CD4 helper T cell activation. A murine model of Borrelia infection will be used to conduct three Specific Aims. Specific Aim #1 will determine the extent to which the early-induced specific IgM and/or IgG responses actively inhibit protective immunity using genetically altered mice unable to secrete certain Ig isotypes. Specific Aim #2 will test whether Bb drives lymph node B cell responses away from germinal center responses. This could result in the lack of affinity maturation and longevity and could cause a loss of functionality over time, as observed in Lyme Borrelioses. Assays will include multicolor flow cytometry to determine the nature and quality of the induced response. Furthermore, the responding B cell subsets will be identified and their regulation by CD4 T cells will be assessed. Also, the extent to which a lack of affinity maturation and development of long-lived plasma cells underlies the functional defect in the Bb-specific antibody responses will be determined by studying antibodies to decarin-binding protein as a prototype response. Specific Aim #3 will determine whether the lack of long- term functional B cell response induction to Borrelia is due to active inhibition and/or a failure of functional CD4 helper T cell response induction or whether an intrinsic defect in the ability of B cells to respond to T cell help underlies the lack of germinal center responses seen in Lyme Borreliosis. Together these studies will elucidate mechanisms that lead to the failure of the vigorous but ultimately deficient B cell response to facilitate clearance from Borrelia infection. PUBLIC HEALTH RELEVANCE: Lyme Disease is a chronic disabling disease resulting from a tick-bite. The tick transfers a bacterium, Borrelia burgdorferi, to the patient, which can persist in various organs, including the joints and heart, where it can cause disabling disease such as chronic arthritis and heart muscle disease. The body develops an immune response that can initially fend off disease but cannot clear the infection. Thus, if infections are not treated continuing disease is common. This proposal aims to determine why the immune response that is sufficient to help to reduce disease ultimately fails to clear the infection. Such information is necessary to develop vaccines and treatments for this increasingly prevalent disease.